Tuning arrangement



M. S. GLASS TUNING ARRANGEMENT Jan. 9, 1951 3 Sheets-Sheet 1 Filed May v2?., 1944 ATTORNEY Jan. 9, 1951 M. s. GLAss TUNING ARRANGEMENT 3 Sheets-Sheet 2 Filed May 22, 1944 /N VEA/TOR M. 5. GLASS ATTORA/FV Jan. 9, 1951 M, s, GLASS 2,537,341

TUNING ARRNGEMENT INVENTOR M. 5. GLASS v WMM A T TOR/VEY Patented Jan. 9, 41951 UNITED STATES OFFICE TUNING ARRANGEMENT `Myron S. Glass, 'West Orange, -N. .Lass'ignorto Bell Telephone Laboratories, IncorporaterLNew York,"N. Y., acorporatonzof New/.York

Application May 22, 1944, Serial No. .536,761

(Cl. Z50-36) 3 Claims.

`affected by the value .of the impedance presented to the resonator by the connected circuits. A `similar depedence of the .resonant frequency upon the connected circuits appears when 'the resonatoris used as a part of an oscillation generating system. In this case .the operating 'frequency of the system as a Whole is dependent ,upon the characteristic properties of .the con .nected circuits as well as those of `the resonator.

It is valso known that the tuning of the system may be changed by varying the impedance of the load or other connected circuit. Tuning systems based upon this ,principle `have 'been employed heretofore but with varying success, 'it having been found that a change in thereactance of the load circuit may affect not only the frequency of the generated oscillations but may at the same time either increase or decrease the amplitude of the oscillations. In general, .the operating frequency and power output of '.anoscillator are functions of the particular value of .a complex impedance'into which the oscilla-- tor delivers power. The exact effect of `Va reactance change or of any impedance change in the circuit depends upon the position in 1the part lof the circuit vat which the changeis made. It will 'be understood, of course, that the connected circuit in a microwave `system is `usually in the nature o'f a transmission line of material'length compared with the operating wavelength. The question will arise, -therefore, as to the location 'inthe transmission'line at'which the impedance Ychange should be made. For tuning purposes, fit is desirable that the v'impedance change shall :affect `jonly `the frequency `and not `Athe amplitude of thenscillations.

= An object of the vpresentfinventioni's to deternmine ka position in the connected Acircuit or transmission line at which 'a variation `of reactance alone will produce'purely.a'frequency change and a variation of resistance purely 'an amplitude aange.;

- z A'iurt-her cbject of the *invention yis 'to ldes-ign -2 a variable reactance tuning element with-"the -`inherent property of stabilizing vthe operating frequency of the system at whatever frequency `value the "tuning element is initially adjusted jto maintain. Y

In the drawings,

Figs. 1 and 2 are schematic diagramsA useful in explaining the operation ofthe invention and its .underlying principles;

Fig. 3 is a View, partly in section and *partly diagrammatic, of a testingsystem useful in practicin'g the invention;

Figs. 4 and 5 are plots of data obtainable wit apparatus as shown in "Fig, 3 1in a system koperated under conditions represented in Figs. 1 and "2;

Fig-6 is aview, partly in section and partly diagrammatic, showing a tuning arrangement according to the invention, employinga-coaxial tuning element;

Fig. '7 is a view, partly in 4section and 4lcvartly diagrammatic, `showing anxembodiment l'of Athe -invention. employing e a tuning element constituting a wave guide or cavity-resonator; and

Fig. `8 is a cross-sectional Viewofthe tuning device shown in Fig. 7. l

Referring to Fig. 1, there is vshown diagrammatically a generator or oscillator Vlil connected to a load I! vthrough a connecting circuit-which is represented in 'two `parts consisting of acoupling device 'l2 and a transmissionline vI-B. "The generator l0 is "assumedto comprise the xusual parts essentialto an oscillating system, namely, a resonator of some kind 'and means "for lmaintaining self-oscillations 4in `the reasonator. The lload 'I l may be of yany type Hcapable -o'f utilizing oscillations"produced bythe generator I0. The coupling device l2 nmaybe inthenatureofan adapter between the generatori-'mand'thetransmission line 1.3. `IElements corresponding nto 'fl-vl), 11,12 and 13, respectively, will generally lbein@ Leluded in atypical microwave system becauseit will not usually be possible or-convenient to connect the generator 'by direct physical 'connection .'to the load. The generator `and "load `may .'beseparatedby a distance of several wavelengths and-the impedance Ycharacteristics of `rthe load wilLnot match the impedance of thegenerator except in a very unusual case. The Aseparation in space between the `generator 4and lthe `load is usually bridgedby a transmission line, generally of Luniform impedance, "The coupling I2 is preferably designed'to effect a smooth impedance transition between the generator iland the line I3. In some cases, another Acoupling device may be'necessarybetween the line I 3 and Ltheload i111,

but for simplicity it may be assumed that the load II is matched to the line I3. The principle of the invention is the same whether the load is directly matched to the line or whether an impedance transforming coupling is inserted therebetween.

In practicing as well as explaining the invention, I use as a reference condition a state of impedance match between the line I3 and the load I I. In this condition there is no reection of waves at the junction between the line and the load. Hence, there are no standing waves in any part of the system to the right of the generator I in the reference condition. The vwaves entering the line I3 from the coupling I2 are pure traveling waves as long as the reference condition is maintained. The system shown in Fig. 1, or any generating system in the reference condition, will be found to have a definite operating frequency and will supply a definite power output, which values can be measured by known means. Furthermore, it will be found that if any change is made in the system whereby a departure occurs from the reference condition, the value of the operating frequency or of the power output or both will change.

A change from the reference condition may be produced, as shown schematically in Fig. 2, by inserting an arbitrary impedance I4 anywhere between the generaor Ie and the load II, or

even inside the generator Il). In order that the effect may appear in the line i3, the impedance I4 is preferably inserted between the line I3 and the load H. The presence of the impedance I4 -produces an impedance mismatch between the line I3 and the load II with consequent reflection of the wave at the junction of the line I3 and the impedance I4. The reflected wave interferes with the forwardly transmitted wave in l the line I3 to produce a standing wave pattern i point xed with respect to the line I3. At the zero point of the scale I8, the amplitude of the wave illustrated is indicated by the point I3.

n In accordance with known wave theory, a pair of adjacent maximum and minimum points, such as I6 and I'I,are separated by a quarter wave length in the line. Accordingly, the difference in positionof the points I6 and I1 on the scale I8 determines the wavelengnh .of the generated wave produced when the impedance I4 has been inserted in the system. In general, as above noted, the wavelength and the operating frequency, withthe impedance I4 inserted, will differ from the values of these quantities in the reference condition. The power output of the system with the impedance I4 inserted may be measured by known methods Vand will generally differ from the normal power in the reference condition.

The system represented in Fig. 2 is capable of yielding fur her information concerning the per- 1 forma-nce of the system with the impedance I4 inserted. This information includes the ratio of amplitude between the maximumand minil mum, which ratio is commonly calledthe standing Wave ratio. A further piece of information consists in the relative position of a maximum or minimum with respect to the scale t8. For example, the position of the minimum Il may be observed and expressed as a number of wavelengths or fractions thereof, as represented by Ich in Fig. 2, where Ic represents the number of wavelengths and fraction thereof between the points I'I and I9.

Fig. 3 represents an experimental system for determining the values 0f a curve such as I5 of Fig. 2. In Fig. 3, the generator IG has been particularized and represented as a magnetron 20, shown conventionally with a multicavity resonator having a coupling loop 2I inserted in one of the internal resonating cavities. An adapter or coupling device 22 is illustrated for joining the coupling loop 2l with a coaxial transmission line, the latter having an inner conductor 23 and an outer conducting sheath 24. A slot 25 running axially with respect to the sheath 24 is provided for inserting a movable probe 26 for sampling the amplitude of the wave in the line at any point over an ex.ended range. The probe 2li may be held in any suitable manner as by means of a slidable cylindrical segment plate 2'! and may be connected to an amplitude measuring device or detector of any known form as represented by a box 28 which may include a visual amplitude indicator such as a meter 29. A scale 3d, corresponding to the scale IB in Fig. 2, may be provided in a suie-ble position to be read in conjunction with a pointer or index 3I attached to the slider 2.

With the system in the reference condition, the slider 2l may be moved to any position along the slot 25 and assuming the measuring device to be functioning, the reading of the meter 29 will not vary regardless of the reading of the index 3| on the scale 30. If, however, an impedance mismatch is provided anywhere to Athe right-of the probe 2B, a standing wave pattern will be set up and the reading of the meter 29 will vary according to the position of the probe 2B and consequently of the index 3l.

The magnetron 23, coupling loop 2l and adapter 22 do not form a part of the present invention but are merely illustrative of an oscillation generator and of a type of ntermediace apparatus which may be present between the generator and the line.v The occasion for the adapter 22 arises from the physical nature of the output circuit and vacuum sealing arrangements which are a part of the particular magnetron chosen for illustration. The loop 2l is extended to form a straight rod 32 which forms the inner conductor of a coaxial line having a conductive sheath 33, the latter serving as a bushing through which .the conductor, 32 enters the resonator of the magnetron. The protruding end of the sheath 33 has a flared and tapered edge 34 to which is Asealed in known manner an insulating tube v35 which may be of glass and which is also sealed to the rod 32 as shown. VThe sheath 32 has a threaded collar 33 which matches a correspond- `are?,an

iprole Yofthe ared vedge A3'4 andthe `collar '3S `to provide a `'stub lineto 'prevent reflection vof the wave lby `the "flared'edge Aand collar 4in progressing from the 'loop `2I to themain lbody of the `transmission line. 'I'he `effecbivelength -o'f the stub line `should be a half-wavelength or `elsean integraliumber of wavelengths plus Aa half -wavelength.

Withthe varrangement shownin Fig. 3, all'the zdata maybe vobtained "for-a curve like curve l5 of Fig. `2 from which maybe derived the ystanding wave ratio, 'the wavelength associated with the ystanding wave pattern, Vand the displacement of the minimum point from the zero line of reference of the scale 30. The wavelength associated with the reference condition 'andthepower 'tive of any oscillation Lgenerating device andthe co'rri'binationof the loop'2l, 'adapter 22 and associated parts, isintended to represent lany substantially reflectionless connection between the generator and the line.

In ythe uimmediate'neighborhood ofthe normal operating frequency `andnormal power of a given system, there is a vuni-que one-to-one correspondence between the pair of values for the frequency and power on the one hand vandthe pair of values for Athe standingwave ratio and the position of a selected `minimum v(or maximum) amplitude on the line. By 'plc-ttingthe :frequency `power values -against the standing wave ratio :minimum position values, -I secure a Vconvenient graphical representation fromwhiclffam able toselect a point or position along the "line -f| 3 Where `the :insertion of a 'pure reactance will Vchange only'the "operating 4frequency or have a `substantially negligible eiect upon the power output of the `'system `over a useful tuning range. 'The chart to which vil have referred, vis mostconvenient if plotted in specialy selected coordinates lin a form comprising what-is 'termed a transmission'line calculator 'in-an 'article lby 'that title -by `P. H. Smith ipublished in Electronica-issue `of Januar-y 1989, pages 29-31, the chart in question appearing as Fig. 3 of "the cited article.4 The transmission line calculatorof'Smit-h `is, reproduced in Figs. Lland 5 here-in in abbreviated form, showing `only a lfew of the coordinate lines of'fthe'background in order -to avoid confusion. The "radiating heavylines in Figs. -4 and 5 represent lines of requal `frequency while the heavy @nearly `circular lines represent `V`lines of equal power. The fainter lines "are the coordinate ilinesfof theSmith calculator and represent respectively resistance changes and react- `ance changes inthe load. The changes are eX- ;pressed relatively with respect to the numerical value of the impedance of the load in the reference condition which impedance is'the character- `istic impedance of the transmission line. lCsnstant resistance contours 'are shown in the diagram as a set 'of circles having their centers on the vertical axis of the `cha'rt'and 'all `rtangent to each othera't the bottom point. Contours of `constant reactance are shown by curves 4radiating upwardly and outwardlylfrom the'fb'ottoin point Tof .'the chart.

Reference may 'be v'had to the cite'd'article in :Electronics for almere complete description `o'f the coordinate vsystem offthe chart. Briey, the center of the chart represents the reference condition of the system, in other words, unity 'stand- 'ing wave ratio, and .normal operating frequency :and power. 4Angl-dardisplacern'errls withrespect iii.

"to the center or vthe "diagramirepresent 'displacements of l.position 'along the transmissionilinein terms of wavelengths with respect to any larbi-I `trary reference position in the line. Circles, concentric with lthe Acenter of the chart, Y corresrmnd vto standing wave ratios diiferent'from unity. For simplicity :in exhibitingand'using the chart, vit is customaryto omit vradial lines denoting Tangu- -lar displacements vand 'concentric circles denoting standing wave ratios. An example will serve "to showhowa frequency power pair of `values may A'be'plotted*against:fa corresponding standing wave ratio "position `pair vof values. Suppose a given operating frequency `and given 'power 'is found to correspond :to a standing `wave ratio of"2 Yand 'that the minimum ipointiislocatedasixteenth of a wavelengthifrom lthe 'zeropoin't of the :scale I8 :or 30. Usingfany radialline A'of Ithe Achart to correspond yto zero `on the scale, ran angle is measured in la clockwise "direction 'one-eighth of a revolution'and `a radial line :drawn ("a Icomplete wavelength correspondsto :two completerevolutions on the chart). Aconcentric circle of proper radius to `represent `a 4standing wave'ratio'of 2 may then be drawnand `at theintersection'of this circlewiththe"secondmentioned radial line 'is the point at which lthe given "frequency power'values are ftobe plotted. By choosing successively rdifferent values "for Vthe impedance Il! Vany ldesired lnurriberfof points may vbe plotted onthe chart. n Theya'lues vof ire- -quencyandpowerwill be found *to 4vbe distributed `vin su-ch manner that contour lines may be drawn `on the chart in the form of vconstant frequency ilines Vand `constant"power lines respectively. "One of the constant frequency lines `willp'ass vthrough the center lof `the chart, this llinerepresenting ithenormal operating frequency, that 'is, the frequency of operation inthe reference condition. Likewise, one 'of the `constant power lines, namevfly, the `line `representing 'normal power, will pass through the center of the chart. Fig. "4 Jshows'a representative plot with the normal frequency "line and the normalpower line labeled.

llig.\-5fshows thesame data represented inFig. A, but withthe contour lines `rotated aboutthe cen- 'ter of the-chart" by'such an `vangle as vto bring the normal 'frequency contour to a substantially ver-- vticalposition at the center ofthe chart, in which case the normal `power line passes 'substantially 'liorizontallythroug'h 'the center point. The transformation of the chart from the stateshown in 54Fig. '4 to'thatshc-wn in Fig. 5 corresponds 'merely to a change `in the position vof the arbitrary zero point on the scale.. Strictly, there is asligll't error zinvolved inthe rotation of the contour lines due -to the fact that'a wavelength is 'thereby taken as an invariable unit independent of the frequency. It 4is contemplated, however, that the frequency changes kwhich are desired andwhich will be included in the range covered by `the'tests are relatively small in proportion to vthejvalue of 'the normal vfrequency and, provided this is true, Athe `change in wavelength will be negligible." fas far as its effect in distorting the vchart is concerned. In a .system in 'which the .illustrated .measurements werev made,the"normal nfrequency -wasin the neighborhood of *3,0001m'egacycles and each radiating line represents va deviation of YA5 'megacycles or one-sixth of kone per cent.

4 My invention is based upon the discovery Ithat the frequency `and power contours, when they have 'been 4properly Vrotated Jas in "5 .fdllow approximately lthe direction of the .constantfre- -fsistance and constant'rreactancecontours respectivel-y This is especially true in the immediate neighborhood of the normal operating frequency.

It will be evident that. there is a relationship between the inserted impedance4 and the resulting frequency such that if the impedance is inserted at the proper position in the line, the insertion of a pure reactance willresult in substantially a lone-quarter the characteristic impedance of vthe line results in a frequency decrease of nve megacycles with negligible change in power.

The angle by which ,the chart is rotated in passing from Fig. 4 to Fig. 5 is a measure of the distance in wavelengths which separates the arbitrary zero point on the scale I8 or 3E) from a critical point on the line at which impedances Vmay. be inserted to secure the desired interdependence between frequency changes and power changes.

. Having found the proper position for the insertion of the tuning element, a transmission system may be designed with a tuning element in the. desired place. It is important to note that the chart may be calibrated for the insertion of .impedance either in series with the line or in parallel with the line. In general, the critical point for insertion of the impedance if once determined on the basis of a series insertion, will be a quarter wavelength distant from a similar critical point at which the insertion of a shunt f or parallel impedance is appropriate.

The tuning element may conveniently be a tuning stub comprising an adjustable lentgh of either ,.a coaxial line or a wave guide. The coaxial tuner `rwill generally act as a parallel tuning element L whereas the wave guide will act as a series element and either will supply a substantially pure reactance While the fuil set of contour lines plotted as in Figs. 4 and 5 is useful in analyzing the koperation of the system and may give valuable information for design purposes, the proper position for the insertion of the tuning element may be determined from a single contour line, namely, the locus of points corresponding to the normal operating frequency. As hereinbefore described, the normal operating frequency is first determined by measuring the operating frequency with the waves, hereinbefore referred to as the reference system in a state substantially free from standing condition. The normal power output may or may not be measured in conjunction with the measure- `ment of the normal operating frequency. The

fsystemmay then be altered as hereinbefore described to produce in succession a plurality of states in which a standing wave pattern occurs in the system. These states of the system are preferably selected so that in each case the operating frequency is in the immediate neighborhood ofthe normal operating frequency, but if this is- .not convenient, the statesv may be selected at random and measurement continued until a plurality of measurements are obtained in the immediate neighborhood of the normal operating frequency. For each such state the standing wave ratio may be measured and also the position of a minimum (or maximum) amplitude .point in the standing wave pattern may be located with respect to the arbitrary reference point in l the'rtransmission line, which polntnhasbeen fil selected as hereinbefore described. The measured values of operating frequency in the neighborhood of the normal operating frequency may then be plotted'as hereinbefore described, against the background coordinate system comprising values of standing wave ratio and position of minimum (or maximum) amplitude as coordinates as in Fig. 4. The locus of points corresponding to the normal operating frequency may then be located and plotted as in Fig. 4, the locus passing through the center of the chart. The locus will usually be found to approximate a radial line. The arbitrary reference point will correspond to another radial line, the upwardly extending vertical radius in Fig. 4. The angle determined between the locus of normal operating frequency and the radius corresponding to the arbitrary reference point determines the distance in the transmission line between the arbitrary reference point and the critical point at which the operating frequency and the power output may be adjusted substantially independently of each other.

The required location of the critical point is of course not dependent upon the use of an angular coordinate, but will in general correspond, in any suitable coordinate system, to the difference between the average value of the position coordinate along the normal operating frequency locus and the position coordinate of the arbitrary reference point. The location may, although usually less conveniently, be determined by calculation without plotting the measuredvalues.

Fig. 6 illustrates the application of a coaxial tuning stub at a predetermined critical position on the transmission line as may be determined in accordance with the principles of the invention. The tuning stub constitutes a means for introducing a substantially pure reactance in parallel with the line. The amount of the reactance introduced and hence the degree of frequency adjustment is controlled by adjusting the position of an annular short-circuiting slider V(il) manipulated in any suitable manner as by a knob 6 I. Adjustment of the slider 60 is found to have negligible effect upon the power output of the system.

vFiga? and 8 show a wave guide type of tuner located a quarter-wavelength farther distant from the generator as compared with the coaxial tuner of Fig. 6. The tuner of Figs. 7 and 8 has a movable reflector i6 which may be manipulated by means of a rod l' I. The sheath 24 hasan annular break or slit 12 for coupling the tuner to the but-they may be utilized also to improve the frequency stability of a system at any given setting of the tuning element. It will be evident from the foregoing explanation and from transmission line theory, that changes in load impedance or other line conditions are effective to change the operating frequency or the power output or both. A

change in load condition which results in a change in operating frequency, an effect commonly referred to as frequency pulling, may be thought of as introducing a change in the roactive component of the impedance measured at the above-mentioned critical point, since it has been shown that when the impedance measurement is made at the critical point, the power output of the oscillator varies with the resistive component while the frequency varies with the reactive oomponent of the measured impedance. Introduction of positive reactance produces a decrease of frequency while addition of negative reactance produces an increase of frequency. Any tuning device introduced at the critical point will afford some degree of frequency stabilization if a change of frequency causes the tuner to introduce an opposing change of reactance. The system will oscillate at the frequency which corresponds to the net reactance in the line produced by the load plus the tuner or frequency stabilizer. Tuning stubs are found inherently to have reactancefrequency characteristics with the correct algebraic sign to result in frequency stabilization. By employing a tuning element which has a large rate of change of reactance with frequency, the tuning element can be made to exert a very considerable amount of stabilization. The tuner may be considered as a second resonator coupled to the internal resonator of the oscillator. By making the tuner relatively more stable than the internal resonator, the stability of the combination may be improved. In a tuning element, a large rate of change of reactance with frequency denotes a high degree of stability.

I have found that a wave guide tuner located with respect to the transmission line in accordance with the invention produces a greater degree of stabilization than is possible with a coaxial tuner. I have found also that the degree of stabilization offered by a wave guide tuner may be made very large by designing the tuner to operate so that the generated frequency approaches close to the cut-off frequency of the wave guide. It is known that near the cut-off frequency of a wave guide, the rate of reactance change with respect to frequency change becomes very great and it is this fact which is utilized in securing a greater degree of stabilization. There is an additional advantage in the use of a wave guide tuner adjusted close to the cut-01T frequency in that the Wavelength in the wave guide is considerably greater near the cut-off frequency so that more precise tuning adjustments may be made. In other Words, the movement of the reflector 70 for a given frequency change is considerably lengthened by operating the tuner close to the cut-01T frequency of the wave guide.

What is claimed is:

1. A tuning arrangement comprising an oscillator the frequency and the power output of which are functions of the particular value of a complex impedance into which the oscillator delivers power, a load device, a transmission line for transmitting power from said oscillator to said load device, means operative initially to substantially eliminate standing waves in said line when said oscillator is supplying power to said load 10 device through said line, and a wave guide branch line coupled to said line for tuning purposes at a point in said line where the effect upon the oscillator of varying the impedance of said tuning means is substantially a maximum of frequency change for a minimum of change in the power output, the said wave guide branch line having a cut-on frequency below, and relatively close to, the desired tuning range, whereby frequency stabilization of the arrangement is promoted at an operating frequency within the desired tuning range.

2. A tuning arrangement comprising an oscillator the frequency and the power output of which are functions of the particular value of a complex impedance into which the o-sciilator delivers power, a load device, a transmission line of material length compared With the operating wavelength of the said oscillator for transmitting power from said oscillator to said load device, means operative initially to substantially eliminate standing waves in said line when said oscillator is supplying power to said load device through said line, and a Wave guide branch line coupled to said line for tuning purposes at a point in said line at least a half wavelength from said oscillator where the elect upon the oscillator of varying the impedance of said tuning means is substantially a maximum of frequency change for a minimum of change in the power output, the said wave guide branch line having a cut-off frequency below, and relatively close to, the desired tuning range, whereby frequency stabilization of the arrangement is promoted at an operating frequency Within the desired tuning range.

3. A coaxial transmission line having an inner conductor mounted within a hollow outer conductor, a hollow Wave guide stub mounted upon said outer conductor, said outer conductor having an annular slit therein opening into said wave guide stub for coupling thereto, and an annular reflector mounted within said stub and surrounding said outer conductor of said coaxial transmission line for breaking up oscillations of harmonics or other undesired modes.

MYRON S. GLASS.

REFERENCES CITED The following references are of record in the file of this patent:

UNITED STATES PATENTS OTHER REFERENCES Hyper and Ultra-High Frequency Engineering, Sarbacher and Edson, John Wiley & Sons, Inc., 1943, pages 354 and 355. 

